Opportunity to train pet armies (5,4)
I believe the answer is:
spare time
'opportunity' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'train pet armies' is the wordplay.
'train' is an anagram indicator (to train can mean to grow something into a shape).
'pet'+'armies'='petarmies'
'petarmies' anagrammed gives 'SPARE TIME'.
'to' is the link.
Can you help me to learn more?
(Other definitions for spare time that I've seen before include "When we can please ourselves what we do" , "Leisure moments" , "leisure hours" , "Prime seat for leisure period" , "Free period, no commitments (5,4)" .)